1. Field of the Invention
This invention relates to marine jet propulsion units, and is particularly directed to a variable area inlet for waterjet propulsors.
2. Description of the Prior Art
It has long been known that the dimensions of the inlet have a substantial effect on the efficiency of waterjet propulsors which varies as a function of the boat speed. To accommodate this effect, it has been suggested to provide means to vary the inlet area so that it could be made smaller, as the boat speed increases. Typically, this is accomplished by providing a lip on the inlet which is movable between an extended portion, supplying low velocity water to the pump at low boat speeds, and a retracted position supplying high velocity water to the pump for high boat speeds. This inverse variation of inlet area to boat speed is necessary because the pump flow rate and velocity remain substantially constant, while the relative inlet velocity of the water varies with boat speed. Unfortunately, the variable inlets of the prior art have had limited diffusion range and have required relatively long axial length in which to achieve diffusion, which decreases the useful space within the boat.